This invention is concerned with circuitry for providing a local loop connection between a telephone central office and a subscriber's premises, and is more particularly concerned with power feed circuitry for the subscriber loop.
It has been proposed to implement a portion of the signal path between the central office and subscriber premises in the form of optical fibers. In such systems, known as "fiber-in-the-loop" (FITL) systems, an Optical Network Unit (ONU) is connected between the subscriber side of the optical fiber and the metal wire pair which completes the loop to the subscriber premises.
It has also been proposed to combine distribution of telephone signals with video signals in a broad-band coaxial cable network. An example of such a so-called cable telephony system is described in U.S. Pat. No. 5,351,234, which has a common inventor with the present application. (The disclosure of the '234 patent is incorporated herein by reference.) In a cable telephony system, a Subscriber-Cable Interface Unit (SCIU) is installed between the coaxial cable and the metal wire pair which completes the subscriber loop.
It is customary to include in the ONU of an FITL system, or in the SCIU of a cable telephony system, a power feed circuit which provides at least some of the power required for the terminal equipment installed at the customer premises. FIG. 1 schematically illustrates such a power feed circuit.
As seen from FIG. 1, the power feed circuit includes a power supply S which is connected to a telephone 20 or other terminal equipment installed at the customer premises through a loop interface unit 22 and a tip and ring wire pair 24 which constitutes the loop connection between the interface unit 22 and the telephone 20.
The loop interface unit provides voice band transmission, ringing, and other associated functions, including determining whether the telephone 20 is in an on-hook or off-hook condition by monitoring the loop current, I.sub.L. The power supply S typically contains a battery or constant-voltage source, which is connected in parallel to serve the respective loops of a number of different subscribers (although only one such loop is shown in FIG. 1). The power supply S also includes a current-limiting resistor for each loop.
According to applicable standards promulgated by Bellcore or ANSI, the loop voltage V.sub.L to be provided by the power feed circuit should be no less than 9 V when the customer premise equipment is in an off-hook condition, and no less than 21 V when the customer premise equipment is in an on-hook condition. The current to be provided for the off-hook condition must be no less than 20 mA, assuming that the loop and customer premise equipment meet the requirement that the off-hook resistance is no greater than 450.OMEGA.. For the on-hook condition the resistance must be at least 5M.OMEGA., so that the requirement for on-hook loop current is only 4.2 .mu.A.
The cost of providing loop power is an important element in the overall cost of the FITL or cable telephony system, and it would therefore be desirable to achieve greater efficiency than that found in conventional battery plus resistor power feed circuits. It is also believed that a power feed circuit capable of supplying substantially more than the minimum power required for the on-hook condition may provide significant advantages. For example, a "loop reconnection device" disclosed in U.S. Pat. No. 5,764,754 (application Ser. No. 08/362,613, filed Dec. 22, 1994, and having a common inventor with this application) might require substantially more power than that provided for in the existing advisory standard for on-hook power. The disclosure of the '613 patent application is incorporated herein by reference.
It is therefore a primary object of the invention to provide an improvement in subscriber loop power feed circuits of the type employed in fiber-in-the-loop and cable telephony systems.
It is an additional object of the invention to reduce the power dissipation and cost of a telephone subscriber loop system in which a specified amount of DC power is required to be supplied to the subscriber equipment when it is in an on-hook condition.
It is a further object of the invention to provide a power feed circuit which is capable of substantially exceeding the recommended criteria for providing on-hook power without adversely affecting off-hook power dissipation.